Surat Al-Shuraa (42:40):
وَجَزَاء سَيِّئَةٍ سَيِّئَةٌ مِّثْلُهَا فَمَنْ عَفَا وَأَصْلَحَفَأَجْرُهُ عَلَى اللَّهِ إِنَّهُ لَا يُحِبُّ الظَّالِمِينَ
And the retribution for an evil act is an evil one like it, but whoever pardons and makes reconciliation –his reward is [due] from Allah. Indeed, He does not like wrongdoers.
What does this mean; that his ajr/reward rests with Allah?
It means; “The pages of the Quran are unable to hold this ajr/reward that is mentioned. That, also the dunya/world and everything within it could not carry this ajr so that it can be mentioned. So Allah (swt) preserved it in Ilm AL-Ghayb [the knowledge of the Unseen] so that he can reward those who forgive/pardon others and make peace with them with an unimaginable reward!